The Heller U.S. Pat. No. 5,119,104, entitled: "Location System Adapted for Use in Multipath Environments," describes a motion-based object tracking system, proposed for use in a multipath environment, such as a semiconductor manufacturing facility, where objects such as integrated circuit wafer boxes are transported among various (processing, inspection and testing) sites by way of an intra plant conveyor. In accordance with the system proposed in the Heller patent, the objects (e.g., wafer boxes) are `tagged` with micro-miniaturized radio TAG transmitters. The operation of a respective TAG is triggered by a motion sensor.
When the wafer box is stationary, the TAG's radio transmitter is in a power-save or quiescent mode, in order to prolong the life of the battery used for the TAG's transmitter circuit. When the wafer box is moved, however, the motion sensor initiates a transmit mode of operation of the TAG, in response to which the TAG's transmitter begins emitting a radio signal encoded with the identification of the tag, and continues to do so in a repetitive and random manner, as long as the object/box is moving.
Using multilateration receivers distributed throughout the monitored area of interest and referenced to a common time base for a time-of-arrival determination processor, the location of a respective TAG and its associated box can be tracked as the box is being moved, up to the point where it is again at rest at the next downstream station. The TAG then reverts to the quiescent or power-saver mode, where transmission is disabled until the box is again moved.
One of the principal shortcomings of the motion-dependent object tracking system proposed in the Heller patent is the fact that, in addition to being motion-dependent, the disclosed system does not effectively solve the problem of multipath inputs to its tracking receiver subsystem, contrary to what the patent alleges, since its receivers are relatively simple amplitude detection devices that operate on the premise that the strongest signal is the first-to-arrive signal. This means that the Heller approach will erroneously identify a later arriving, relatively large amplitude, multipath signal that may have passed through an open window and reflected off a highly reflective surface, such as a nearby water tower, without significant attenuation, over a relatively weak, but first-to-arrive signal, that has travelled in a direct path from the TAG to the receiver, but has had to pass through an attenuating medium, such as a wall to reach the receiver.
A further shortcoming of the object tracking system proposed in the Heller patent is the fact that it is not concerned with the more fundamental problem of asset management. Asset management not only addresses the need to locate and track processed components in the course of their travel through a manufacturing and assembly sequence, but is also concerned with the more general problem of component and equipment inventory control, where continuous knowledge of the whereabouts of any and all assets of a business, factory, educational, military or recreational facility, and the like, is desired and/or required. Moreover, an asset management system may benefit from status information that can be provided to the tag by means of an auxiliary sensor associated with the tag, something not address by the Heller scheme.